CI: disable KAS_REPO_REF_DIR by default
Having local repo caches is a little fiddly to manage, and by definition
we're running CI inside GitLab which supports mirroring repositories
automatically.
As these mirrors are always available and update automatically, make
Kas reference directories opt-in and instead expect that the site is
either fine with full fetches, or is using KAS_PREMIRRORS.
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>
Ross Burton